Anubis/Atum stretch vector signatures

   
 Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

This photo corresponds to the area of the so-called Anubis missing slab. It’s the area the slab originally sat on before departing from the comet. It’s on the body lobe and the duck is upside-down in this view. The slab actually comprises Anubis and Atum. However, Atum wasn’t very visible when matches were being made to the head lobe. Those matches were from Anubis to the head rim as it would have been when seated along the Anubis border (Part 17). Those matches betrayed the missing slab so it was called the Anubis missing slab. 

Key to photo:

Blue- today’s rotation plane or equator. Also known as the xy plane which is the plane through which both the x and y axes sweep like four spokes as they rotate about the z rotation axis. 

Brown- the longer brown line is the paleo rotation plane. That’s the rotation plane that held sway during stretch before head shear and would have influenced the purported stretch signatures in this photo. See the relevant paleo plane ‘page’ in the blog menu bar. The three brown dots in the bottom-left corner denote the estimated paleo z axis or axis of rotation. Of course, it’s at 90° to the paleo plane.

Three red dots in shadow- these are delaminated sections of crust on the ‘red triangle’ on Seth. These are crust delaminations similar to the red triangle recoil (Part 26, signature 6). They haven’t yet had their own post. 

Fuchsia- the main onion layers exposed by the slab loss on Anubis/Atum. Massironi et al defined them as onion layers (but it has to be made clear that they didn’t invoke slab loss or stretch). These onion layers are on Atum because Atum is rougher and Anubis is smoother. They appear to have delaminated towards the long axis extremity which is the exact point where the brown paleo line disappears behind the middle of Apis (Apis is the very straight section that the paleo line bisects).

Small red- three lines that appear to join the three red triangle delaminations to three of the fuchsia onion layers, lending weight to the idea that the fuchsia onion layers delaminated under stretch. That’s because the red triangle delaminations have already been mirror-matched across their faces (in shadow here). That means they are definite delaminations as opposed to being stacked ‘slabs’ that might have delaminated as in the case of the fuchsia onion layers or the so-called ‘gills’ below them (below in ‘upright duck’ view or above here because the duck is upside down).

Bright green, orange and upper red lines of dots- these are suggested delaminations. The green line includes the so-called gills, three obvious sticking-up slabs. There are also three very tiny green dots to the left of the gills. They’re smaller than all the others for fear of obscuring the tiny nested triangles that are in line with the gills. 

Notice that all the suggested delaminations are long, straight lines which are exactly in line with the direction of the potential tensile force vectors of stretch i.e. towards the long axis extremity and as parallel as is feasible to the paleo rotation plane. So their direction of stretch is as faithful as could be expected to the behaviour they would exhibit under stretch.

It’s useful to rotate the photo clockwise so that the lines are sloping down to the right. If this were a landslide then gravity would be working downhill to the right and the various rocks and layers would be delaminating in this direction. The gradation of resistance to sliding would likely increase with depth unless it was a clean shear at one deeper layer only. In other words, lower layers would move slightly more slowly than upper layers. That means any solid slab that started out sticking downwards into the matrix to some extent would have a tendency to be more anchored at its buried end. This would cause its exposed end to succumb to the tensile stretch force more than its buried end. That would make it tip up more and more vertically. This may be the reason the three ‘gill’ slabs are sticking upwards somewhat. That may also apply to other similar features further along the gills’ delamination line and on other lines. 

This photo shows exactly one side of the body. And this side is one of the four equal-length sides of the diamond-shaped body. The paleo rotation plane (and current plane) run through the long diagonal and the rotation axis runs across the short diagonal, as you’d expect with a stretching comet. The delaminations only start halfway across this side. This is in keeping with the increasing centrifugal forces across the surface towards Apis. The closer to the long axis extremity at Apis, the higher the forces, and the higher the consequent delamination. The flatter area nearer to the three brown dots of the rotation axis wouldn’t be expected to stretch as much, being at a lower radius from the axis. So it wouldn’t delaminate. But it would still have been attached to the area that was stretching to delamination and was probably yanked away with that stretched section when it escaped. 

The direction of the delaminations was said above to be in keeping with the tensile force vectors of stretch. Another way of putting it is that the delaminations run away radially from the south pole (insofar as the south pole position can be estimated with no shape model showing it yet). That’s because they run directly away from the paleo z axis dotted in brown at bottom left of the header. So this is the first example of radial stretch vectors centred on the south pole instead of the north pole. It supports Parts 32 to 36 which show delaminating and sliding crust all with tensile force vectors pointing out radially from the north pole.